1. Field of the invention
The present invention is generally concerned with clutches and it is more particularly directed to clutch cover assemblies, that is to say the assembly of parts which is to be attached to a reaction plate or flywheel with a friction disk inserted between the two to form a clutch, in particular a clutch for automobile vehicles.
2. Description of the Prior Art
A clutch cover assembly comprises a first generally annular part called the cover adapted to enable the assembly to be attached to a reaction plate or flywheel, a second generally annular part called the pressure plate axially movable relative to but constrained to rotate with the cover, and engagement means adapted to urge the pressure plate in the axial direction away from the cover so as to press the friction disk disposed between it and the reaction plate against the reaction plate.
The engagement means enable the clutch to be engaged and are associated with disengagement means enabling the clutch to be released. They are usually spring means.
They may be axially acting spring means comprising at least one Belleville washer, for example.
This is the case with French Pat. No. 2 441 762.
It is also the case with clutch cover assemblies using diaphragm springs, that is to say clutch cover assemblies in which a third generally annular part called the diaphragm spring has a circumferentially continuous peripheral part forming a Belleville washer and a central part subdivided by slots into radial fingers; these fingers form clutch release levers constituting the associated disengagement means and adapted to procure progressive engagement and release of the clutch.
One advantage of a diaphragm spring of this kind, quite apart from the simplicity of implementation resulting from its use, is to introduce a demultiplication effect due to the lever arm of the fingers so that, other things being equal, it is possible to reduce the force to be applied to the ends of the fingers to release the clutch.
It is to be understood that when the engagement means are also formed by axially acting spring means more than one Belleville washer may be provided, at least two, for example.
As a general rule the Belleville washers are provided with respective inside and outside edges of the same diameter as each other and, according to how they are disposed within the clutch cover assembly concerned, they operate in series or in parallel.
At present, whenever the engagement means comprise at least one Belleville washer there is always a radial offset between the circumference on which a Belleville washer is axially supported and that on which it presses on the pressure plate in order to engage the clutch.
One edge bears on the cover or some form of spacing member, in practise fixed relative to the cover, in the axial direction and either directly or indirectly, for example; its other edge acts either directly or indirectly on the pressure plate, and the radial offset between the respective bearing and supporting points therefore corresponds to the radial distance between these edges.
If in order to minimize the overall radial size of the assembly use is made of a Belleville washer having the same diameter as the pressure plate, and generally superposed on the pressure plate, and if no specific provision is made to the contrary, the force exerted by the Belleville washer on the pressure plate is applied on a circumference the diameter of which is close to that of one or the other of two circumferences delimiting the active area of the pressure plate, that is to say the part of the pressure plate through which it is adapted to bear axially on the corresponding friction facing of the associated friction disk.
An arrangement such as this inevitably leads to uneven wear of the friction facing, to the detriment of proper utilisation of the friction facing and of proper functioning of the assembly.
To alleviate this disadvantage it has been proposed to arrange for the Belleville washer to exert its force on the pressure plate on a circumference the diameter of which is substantially intermediate the diameters of the circumferences delimiting the active area of the pressure plate.
In practise the arrangement currently adopted to meet this requirement simply entails having the Belleville washer bear on the median area of the pressure plate, a like arrangement being adopted when two Belleville washers are employed in series with each other.
However, if no specific provisions are made to the contrary, this entails offsetting the Belleville washer or washers employed relative to the pressure plate in the radial direction.
If this radial offset is in the direction towards the axis of the assembly, there result difficulties with fitting the Belleville washer or washers concerned allied with an overall reduction in their size, to the detriment of the loads that they are capable of transmitting.
If, on the other hand, the radial offset is in the direction away from the axis of the assembly, the inevitable result is a general increase in the overall diameter of the assembly and an increase in the lever arm whereby the Belleville washer or washers employed bear axially against the cover, with the risk of the cover flexing.
This is why it is usually proposed to reduce the overall dimensions of the Belleville washer or washers relative to those of the pressure plate so that, although they bear axially on a median area of the pressure plate, they remain generally within its overall contour.
Although it may prove satisfactory from the point of view of the overall diameter, this compromise is nevertheless achieved to the detriment of the loads that the Belleville washer or washers employed are able to withstand.
Other things being equal, these loads depend in particular on the diameters of the circumference on which the Belleville washers bear.
A general object of the present invention is an arrangement that makes it possible to meet the requirement in relation to bearing engagement with the pressure plate without increasing the overall diameter of the assembly.